Light and electron microscopic observations on cell division in two large pennate diatoms. Hantzschia and Nitzschia. II. Ultrastructure.

Cells, preselected to cover all stages of mitosis, were sectioned accurately for investigating changes in spindle structure that accompany mitosis. During spindle formation, the interphase Microtubule Center (MC) breaks down. Numerous tiny foci, each apparently nucleating one microtubule (MT) and derived from the MC, line up along the two polar plates; lateral interaction between these two sets of (oppositely polarized--?) MTs is presumed to generate the MT packing arrangement characteristic of the diatom spindle's overlap. Later, when the elongating central spindle enters the nucleus at prometaphase, the MTs from each polar plate have either interacted thus to generate the central spindle proper, or else they radiate into the nucleus. This latter population of MTs interacts with the kinetochores and most become thereby organized into kinetochore fibres. The zone of overlap quickly develops ragged edges, suggesting that it is labile (i.e., by irregular sliding and/or growth of MTs) even at early prometaphase. Metaphase spindle structure is as expected from light microscopy. The collar material is difficult to discern, but it apparently permeates the kinetochore fibres. During anaphase, the overlap diminishes and disappears as the spindle elongates. The chromosomes always move past the ends of the spindle, a movement accomplished without any apparent involvement of MTs. In N. sigmoidea, the spindle invariably breaks down upon completion of elongation, and the scattered remnants of its MTs soon disappear. In contrast, the central spindle of H. amphioxys persists until it is broken by the cleavage furrow; the MTs in the half spindle away from the overlap always exhibit pronounced clumping. These observations are integrated with extensive observations on mitosis in vivo, with a view to understanding the mechanisms of spindle formation, function and disassembly.